1. Field of the Invention
The present invention relates to methods for manufacturing a reflection type liquid crystal display (LCD) and a reflection and transmission composite type LCD, and more particularly, to methods for forming a photosensitive insulating film pattern and a reflection electrode each having an uneven upper surface, and a method for manufacturing a LCD having the reflection electrode using the same.
2. Description of the Related Art
In an information-oriented society these days, the role of an electronic display is becoming more important. All kinds of electronic displays are widely used in various industrial fields. As techniques of the electronic display field are continuously developed, various electronic displays having new functions are provided corresponding to diverse requirements of the information-oriented society.
Generally, an electronic display is an apparatus for visually transmitting information to a person. That is, an electronic display can be defined as an electronic apparatus, which converts an electrical information signal output from various electronic equipments into a visually recognizable optical information signal. Also, it may be defined as an electronic apparatus serving as a bridge for connecting the person and the electronic equipment.
These electronic displays are classified into an emissive display, in which the optical information signal is displayed by a light-emitting method, and a non-emissive display, in which the signal is displayed by an optical modulation method such as light-reflecting, dispersing and interference phenomena, etc. As the emissive display is called an active display, a CRT (cathode ray tube), a PDP (plasma display panel), an LED (light emitting diode) and an ELD (electroluminescent display), etc. may also be mentioned. As the non-emissive display is called a passive display, an LCD, an EPID (electrophoretic image display), etc. may also be mentioned.
The CRT has been used in an image display device such as a television receiver and a computer monitor, etc., over the longest period of time. The CRT has the highest market share in an aspect of displaying quality and economical efficiency, but it also has many disadvantages such as a heavy weight, a large volume and high power consumption.
Meanwhile, as various kinds of electronic devices are small-sized and lighter in weight, along with the solidification and lower voltage and lower driving power of the electronic devices due to rapid advancement of semiconductor technologies, there is requested a flat panel type display having slimmer and lighter properties as well as lower driving voltage and lower power consumption characteristics according to the novel environment.
Among variously developed flat panel type displays, the LCD is much slimmer and lighter than any other displays and it has lower driving voltage and lower power consumption. Also, the LCD has the displaying quality similar to that of the CRT. Therefore, the LCD is widely used in various electronic devices. Further, since the LCD can be easily manufactured, its application is becoming gradually wider.
The LCD is classified into a transmission type LCD, which displays an image using an external light source and a reflection type LCD, which displays the image using ambient lights instead of the external light source.
The reflection type LCD has an advantage in that it consumes less power and shows an excellent display outdoors as compared with the projection type LCD. Further, the reflection type LCD is thin and light because an additional light source such as a backlight apparatus is not necessary.
However, the current reflection type LCD has a dark screen and fails to show high definition and multicolor images. Therefore, the reflection type LCDs are restrictively employed for a product that requires a simple pattern display, such as numbers or simple characters.
To use a reflection type LCD for various electronic displays, a high definition and a multicolor display together with an enhanced reflection luminance are necessary. In addition, proper brightness, rapid response speed and enhancement of contrast are necessary.
In current reflection type LCDs, two technologies are combined for an enhancement of the brightness. One is enhancing the reflection efficiency of the reflection electrode, and the other is achieving an ultra high aperture ratio. Naofumi Kimura discloses a method of enhancing the reflection efficiency by forming bumps to a reflection electrode in U.S. Pat. No. 5,610,741, issued Mar. 11, 1997, entitled xe2x80x9cReflection Type Liquid Crystal Display Device with Bumps on the Reflector.xe2x80x9d
FIG. 1 is a partial plan view of the reflection type LCD device provided in the above U.S. Patent, and FIG. 2 is a sectional view of the reflection type LCD device of FIG. 1.
Referring to FIGS. 1 and 2, the reflection type LCD device is comprised of a first substrate 10, a second substrate 15 facing the first substrate 10 and a liquid crystal layer 20 interposed between the first and second substrates 10 and 15.
The first substrate 10 includes a first insulating substrate 30 on which a plurality of gate bus wirings 25 is formed. Gate electrodes 35 branch off from the gate bus wirings 25. Additionally, a plurality of source bus wirings 40 are provided so as to be orthogonal with and maintain the insulation from the plurality of gate bus wirings 25 due to an insulating layer between the source bus wirings 40 and the gate bus wirings 25. Source electrodes 45 branch off from the source bus wirings 40.
Reflection electrodes 50 are formed between the first substrate 10 and the liquid crystal layer 20 and are disposed in a plurality of rectangular regions formed by crossing the plurality of gate bus wirings 25 and the plurality of source bus wirings 40.
The reflection electrode 50 is connected with a thin film transistor (TFT) device 55 formed on the first substrate 10, the TFT device 55 serving as a switching device with the gate bus wiring 25 and the source bus wiring 40.
A plurality of dents 70 and 71 are provided on the surface of the reflection electrode 50, whereby the surface is made bumpy. The plurality of dents 70 and 71 are irregularly arranged on the entire surface as depicted in FIG. 1. The reflection electrode 60 and a drain electrode of the TFT device 55 are connected to each other through a contact hole 65.
The gate bus wiring 25 and the gate electrode 35 are formed on the first insulating substrate 30 made of, for example, glass by depositing tantalum (Ta) film using a sputtering method and patterning the deposited Ta film using an etching or a photolithography process.
Next, the gate insulating film 75 is formed to cover the gate bus wiring 25 and the gate electrode 35. The gate insulating film 75 is made, for example, by forming a 4000 xc3x85 thick SiNx film by a plasma CVD (Chemical Vapor Deposition) method.
Referring to FIG. 2, a semiconductor layer 80 of amorphous silicon (a-Si) is formed on the gate insulating layer 75 on the gate electrode 35. Contact layers 85 and 90 of n+ type impurities-doped a-Si layer are formed on the semiconductor layer 80.
Subsequently, molybdenum (Mo) film is formed on the first insulating substrate 30 to cover those members formed in the above-mentioned manner and then the Mo film is patterned to form a source bus wiring 40, a source electrode 45 and a drain electrode 60. In such a manner, a TFT device 55 including the gate electrode 35, the semiconductor layer 80, the contact layers 85 and 90, the source electrode 65 and the drain electrode 60 is completed.
On the entire surface of the insulating substrate 30 in which the TFT element 55 was formed, an organic insulating film 95 and a reflection electrode 50 each having a bumpy surface are sequentially formed.
FIGS. 3A and 3B are sectional views showing the steps of a method for forming bumps in the device shown in FIG. 2.
Referring to FIG. 3A, a resist film 100 is formed on the surface of the first insulating substrate 30 by a spin coating method to cover the metal pattern 55 of aluminum (Al) or nickel (Ni) with a high reflectivity. The metal pattern 55 includes, for example, the source electrode, the drain electrode or the storage electrode for the TFT. Thereafter, the resist film 100 is pre-baked.
Next, a mask 110, where a light transmitting region 105 and a light shielding region 106 are formed in a predetermined pattern, is arranged over the coated resist film 100 and then exposure and development processes are carried out so that bumps 115 corresponding to the pattern of the mask 110 are formed as shown in FIG. 3B. When a thermal treatment of the substrate is carried out, a bump 115 whose angles are rounded off is formed.
Returning to FIG. 2 again, an organic insulating film 95 is applied to cover the bumps 115, for example, by the spin coating method and thereby the surface of the formed organic insulating film 95 becomes bumpy due to the bumps 115.
Subsequently, in the reflection type LCD as shown in FIG. 2, the organic insulating film 95 is patterned using a mask (not shown) to form a contact hole 65 exposing a surface of the drain electrode 60 of the TFT device 55. The contact hole 65 is filled with the reflection electrode material. The reflection electrode material is formed by the vacuum deposition method. Resultantly, dents 70 and 71 are formed in the surface of the reflection electrode 50 such that they have shapes corresponding to those of the organic insulating film 95.
Afterwards, a first orientation film 120 is formed on the reflection electrode 50 and the inorganic insulating layer 95, whereby the first substrate 10 is completed.
The second substrate 15 includes a second insulating substrate 140 on which color filters 125, a common electrode 130 and a second orientation film 135 are formed.
The second insulating substrate 140 is made of glass. Color filters 125 corresponding to the unit pixels are formed on the second insulating substrate 140. On the color filters 125 is formed a common electrode 130 made of a transparent material such as indium tin oxide (ITO). A second orientation film 135 is formed on the common electrode 130, whereby the second substrate 15 is completed.
The second substrate 15 is arranged to face the first substrate 10 and then the liquid crystal layer 20 including a liquid crystal material 21 and a pigment 22 is injected into a space between the first substrate 10 and the second substrate 15 by a vacuum injection method, whereby the reflection type LCD is completed.
Another conventional method for forming the aforementioned bumpy structure is to use a photosensitive organic insulating film. This method enables formation of the insulating layer with the bumpy surface structure only by using one kind of material layer instead of using the two layers of the resist film 100 and the organic insulating film 95 as explained in FIGS. 2, 3A and 3B. In other words, the photosensitive organic insulating film is coated instead of the resist film 100 shown in FIG. 3A. A conventional photolithography process against the photosensitive organic insulating film is carried out, whereby bumps, dents and the contact hole are formed. Thereafter, the resultant substrate is transferred into a subsequent process of the reflection electrode forming process.
However, according to the conventional methods of manufacturing the reflection type LCD, although the plurality of dents formed in the reflection electrode increase the reflection efficiency, they causes some problems as follows.
Referring to FIGS. 3A and 3B, in the above method, an irregular surface structure including the bumps 115 and the dents 117 is formed on a surface of the resist film 100 before the reflection electrode is formed. Then, since the patterns 57 such as the source electrode, the drain electrode and the storage capacitor electrode formed at a lower portion of the resist film 100 in a unit pixel region, is formed of the metal having a high reflectivity, and a space d2 between the light shielding patterns 112 of the mask 110 on the metal pattern 57 is the same as a space d1 between the light shielding patterns 112 of the mask on a portion in which the metal pattern 57 is not located, the light 83 is reflected upwardly from the upper surface of the metal pattern 57 during the exposing process for forming the dents 117. Therefore, as shown in FIGS. 3B and 4, the dent 117 having a diameter larger than a desired diameter is formed on the resist film 100, or it is exposed more deeply to the light than other portions. Worst of all, the dent portion is completely exposed to the light, so that an undesired portion is exposed.
In order to prevent the exposing problem of the undesired portion, an insulating film has to be further formed at a lower portion of the resist film 100. Therefore, the manufacturing process is more complicated and also the manufacturing cost is increased.
In addition, according to the aforementioned conventional reflection type LCD, the hemispherical dents as the micro-lenses, each of which has a different size, are formed so as to increase the reflection efficiency. However, a ridge portion (i.e., bumps) where the dents are not formed in the reflection electrode has a different size depending on its position. Therefore, there is a problem in that the uniformity of the reflectivity of the entire reflection electrode is deteriorated. That is, since the sizes of the portions in which the dents are not formed are different, respectively, the regions in which the sizes of the dents formed on the reflection electrode are different, respectively have different heights. Thus, since the reflection electrode has a different reflectivity depending on the regions, the uniformity in the reflectivity of the reflection electrode is deteriorated. As described above, the deterioration in the reflection uniformity of the reflection electrode causes orientation of the liquid crystal material to be non-uniform, so that a contrast of an image is degenerated. Further, there is a high probability that the non-uniformity of the orientation of the liquid crystal material generates a fog failure as well as an afterimage due to leaked light.
In an actual manufacturing process, since the sizes of the dents formed in the reflection electrode and the sizes of the regions between the dents are different from each other, there is a disadvantage in that it is substantially very difficult to precisely control the sizes of the dents and the spaces between the dents in accordance with design values.
Moreover, although the dents having the different sizes are formed to be overlapped with each other, since they have a hemispherical shape, it is very difficult to completely prevent scattered reflection of the incident light at the dents portion. Therefore, there is a limitation to improve the quality of the image.
Also, the conventional reflection type LCD basically has a foursquare pixel shape. However, as a great variety of information communication equipment, such as a portable cellular phone and an LCD TV, etc., are developed recently, various pixel sizes are requested. If a pixel having a desired size has to be applied to a display device requiring a different pixel size, the display device should be redesigned from the beginning. Also, there is a problem that a condition of the manufacturing process has to be secured again. Particularly, in case of an electronic display device such as the portable cellular phone, which is required to have a high reflectivity in a specific direction, it is further difficult to apply the pixel having the foursquare shape.
Therefore, it is a first object of the present invention to provide a method for forming a photosensitive insulating film having an uneven surface of uniform prominences and recesses.
It is a second object of the present invention to provide a method for forming a reflection electrode film having an uneven surface of uniform prominences and recesses (or protrusions and dents).
It is a third object of the present invention to provide a method for manufacturing an LCD having a reflection electrode that is especially suitable for manufacturing the LCD including the reflection electrode film having an uneven surface of uniform prominences and recesses.
It is a fourth object of the present invention to provide a method for manufacturing an LCD having a reflection electrode that has an uneven surface of uniform prominences and recesses for allowing the reflection electrode to have the same reflectivity throughout the entire region of the reflection electrode.
To achieve the first object of the present invention, there is provided a method for forming a photosensitive insulating film having an uneven surface of uniform prominences and recesses. In the above method, the photosensitive insulating film is formed on a substrate on which a first electrode having a reflection property is formed. The photosensitive insulating film is exposed to a light. The exposed photosensitive insulating film is developed. Here, a first light amount of the light scanned between first patterns corresponding to an upper portion of the first electrode is different from a second light amount thereof scanned between second patterns corresponding to a portion other than the first electrode.
To achieve the second object of the present invention, there is provided a method for forming a reflection electrode film having an uneven surface of uniform prominences and recesses. In the above method, a photosensitive insulating film is formed on a substrate on which a first electrode having a reflection property is formed. The photosensitive insulating film is exposed to a light. The exposed photosensitive insulating film is developed to form a surface film having an uneven surface of uniform prominences and recesses. The reflection electrode having an uneven surface corresponding to the surface of the photosensitive insulating film is formed on the photosensitive insulating film. Here, a first amount of light scanned between first patterns corresponding to an upper portion of the first electrode is different from a second amount of light thereof scanned between second patterns corresponding to a portion other than the first electrode.
Further, to achieve the third and fourth objects of the present invention, there is provided a method for manufacturing an LCD. In the above method, a photosensitive insulating film is formed on a first substrate on which a first electrode having a reflection property is formed. The photosensitive insulating film is exposed to a light. The exposed photosensitive insulating film is developed to form an uneven surface with uniform prominences and recesses. The reflection electrode is formed on the photosensitive insulating film. A second substrate having a transparent electrode facing the first substrate is formed. A liquid crystal layer is sandwiched between the first substrate and the second substrate. Here, a first amount of light scanned between first patterns corresponding to an upper portion of the first electrode is different from a second amount of light thereof scanned between second patterns corresponding to a portion other than the first electrode.
According to the present invention, dents (recesses) having a uniform width and depth are formed throughout the entire surface of a photosensitive film regardless of the existence of the metal pattern having a high reflectivity at the lower portion of the photosensitive insulating film and thereby a reflection type LCD with an improved reflection efficiency and remarkably improved contrast and picture quality compared with the conventional reflection type LCD can be realized. In addition, since the reflection electrode is formed using the improved exposing and developing process, the manufacturing time and costs are substantially reduced.